With increasingly reinforced environmental consciousness, many countries have limited sulfur content of diesel fuel to a quite low level (10 to 15 μg/g), to reduce the emission of harmful gases, and improve air quality by means of environmental legislation. The sulfur content of diesel fuel was decreased to 15 μg/g in 2006 in USA. The sulfur content of diesel fuel was decreased to 10 μg/g in January, 2003 in Germany. The sulfur content of diesel fuel was decreased to 10 μg/g in 2008 in other countries of European Union (EU) and Japan. The national standard on the urban automotive diesel fuel of China, GB 19147-2009 was formulated according to the Europe's category III standard, which requires the sulfur content to be less than 350 μg/g. In the discharging index of the fifth stage for the automotive gasoline and diesel oil, issued by the State Ministry of Environmental Protection of China, it is required that the sulfur content in the diesel oil should be less than 10 μg/g.
The requirement for the diesel oil in the international market has been increasing constantly. However, the supply amount of high-grade feedstock oil has been decreasing. How to utilize the low-grade feedstock oil to manufacture diesel with ultralow sulfur content to meet the increasing demand has been a big challenge that the refineries have to face. To meet the challenge, on one hand, it needs to resolve the key technological difficulties, and newly construct hydrogenation apparatuses to conduct deep hydrodesulfurization for diesel; on the other hand, it needs to reduce risks and repeated investments in order to guarantee economic benefits. In the traditional hydrogenation process using a fixed bed, it is necessary to transfer hydrogen gas from gas phase to liquid phase, then the reaction between the dissolved hydrogen and sulfur compound occurs at an active centre of catalysts, thereby achieving the desulfuration purpose. During the process, the required volume of hydrogen gas is much greater than that consumed in the hydrogenation reaction. This is because, on one hand, the hydrogenation reaction is a strong exothermic reaction, which, for controlling the reaction temperature, needs a large amount of hydrogen gas and feedstock oil to pass through catalyst beds to bring away reaction heat; on the other hand, in the reaction related to three phases of gas, liquid and solid, maintaining a high hydrogen partial pressure is helpful for the hydrogenation reaction, inhibiting the production of coke and prolonging catalyst life. In addition, the hydrogen not reacted in the reaction may be retransferred to the reactor for reaction after the pressure thereof is increased by passing through a circulating hydrogen compressor. As key equipment for the hydrogenation process, the circulating hydrogen compressor has a high cost in investment and operation. In order to eliminate the circulating hydrogen and the circulating hydrogen compressor so as to reduce the investment cost of the apparatus, a liquid-phase hydrogenation technology was proposed. In the liquid-phase circulation hydrogenation process, the hydrogen gas is premixed with the feedstock oil, so that the hydrogen gas is dissolved in the feedstock oil, then the mixture is introduced into the reactor for reaction. The hydrogen gas needed during the reaction are entirely from the dissolved hydrogen without additional supplement of cool hydrogen. The liquid-phase circulation hydrogenation process possesses advantages of small reactor, low investment cost, easy control of the reaction temperature and the like. However, this liquid-phase circulation hydrogenation process also has one problem, that is, in order to meet the hydrogen gas volume needed in the hydrogenation reaction, it is necessary to use a large amount of circulation oil or additionally add a solvent to dissolve the hydrogen gas, which results in the reduction of the hydrogenation efficiency.
In the conventional fixed-bed hydrogenation process, in order to remove impurities such as sulfur, nitrogen, oxygen, and metal in the raw material or reduce the size of the feedstock oil molecules, it is necessary to carry out a catalytic hydrogenation reaction. In order to control the reaction temperature of a catalyst bed and prevent the catalyst from deactivation by carbon deposition, generally a high hydrogen-oil volume ratio is used, which certainly causes a large amount of surplus hydrogen gas after completion of the hydrogenation reaction. The surplus hydrogen gas is usually pressurized by a circulating hydrogen compressor and then mixed with new hydrogen to be reused as hydrogen feed for reaction. This process can be also defined as a gas-phase circulating, fixed-bed hydrogenation process. For this process, the investment of the circulating hydrogen compressor occupies a relatively high proportion of the whole hydrogenation device cost. The heat exchange system for hydrogen gas has a high energy consumption, so if the hydrogen gas flow in the process of hydrogenation can be reduced and the hydrogen gas circulating system and the circulating hydrogen compressor can be omitted, then the investment of enterprises can be saved, and the cost is reduced for clean fuel production.
Generally, for raw materials containing simple sulfide, the reaction rate of its hydrodesulfurization in a fixed bed hydrogenation reactor is not only related with the concentration of organic sulfide, but also affected by the factors such as wetting situation of the catalyst, concentrations of organic nitride and H2S in the reactor system, and the like. The wetting factor of the catalyst is a measurement of the infiltration degree of the catalyst surface by a liquid reactant under the condition of hydrogenation reaction. The higher is the infiltration degree of the catalyst, the higher the wetting factor of the catalyst will be, that is to say, the higher the effective availability of the catalyst will be. Under the condition that the factors including the catalyst and the like have been defined, the main factor affecting the wetting factor of the catalyst is the flow rate of the liquid in the reactor and the ratio of the flow rate of the gas to that of the liquid. It is generally considered that the increase in the flow rate of the liquid enhances the wetting effect of the catalyst, while the conventional hydrogenation process mostly utilizes a high ratio of hydrogen to oil much more than the need of the reaction, thereby decreasing the wetting effect of the catalyst, and having an adverse influence on the wetting factors. In addition, the investment of the section of hydrogen circulation during petroleum refining occupies a large proportion of the cost of the whole process.
Organic nitride is poison for the hydrogenation catalyst, which has an obvious inhibition effect on the reaction of hydrodenitrification, hydrodesulfurization, and hydrodearomatization. This inhibition effect is mainly caused by the very strong adsorption energy of some nitrides and most of the intermediate reaction product of nitrides to the active centre for hydrogenation reaction of the catalyst, which inhibits occurrence of the other hydrogenation reaction from a viewpoint of competitive adsorption. However, the impurity content of the raw material will be decreased greatly by circulating the hydrogenated product, which helps to exert the performance of the catalyst.
United State patent US20060144756A1 discloses a control system method and apparatus for two phase hydroprocessing. In a continuous liquid phase hydroprocessing, the circulating hydrogen is eliminated, and all the hydrogen needed by the hydrogenation reaction comes from the hydrogen dissolved in the liquid phase without the need of additional hydrogen gas. However, it needs to utilize a solvent or diluent with a high hydrogen solubility to dissolve hydrogen, which influences the subsequent hydrogenation efficiency.
U.S. Pat. No. 6,213,835, U.S. Pat. No. 6,428,686, and CN200680018017.3 discloses a hydrogenation process with dissolving hydrogen in advance, which controls the volume of liquid or the air pressure in the reactor by controlling the hydrogen gas amount contained in liquid feed. But these patents have not totally resolved the problem of removing the detrimental impurity such as H2S and NH3 produced in the hydrorefining reaction process, which results in a continueous accumulation of these impurities in the reactor, thereby greatly reducing the reaction efficiency, and failing to effectively handle raw material with high sulphur and nitrogen content. The above patents do not disclose the specific structure of the reactor, either.
The Chinese patent CN86108622 discloses a hydrorefining process for oil produced by cracking, wherein the volume ratio of hydrogen to oil is 200:1 to 1000:1; the Chinese patent CN93101935.4 discloses a one-stage hydrocracking method for poor-quality feedstock oil, wherein the volume ratio of hydrogen to oil is 1300:1 to 1500:1; the Chinese patent CN94102955.7 discloses a hydrorefining method for DCC gasoline, wherein the volume ratio of hydrogen to oil is 150:1 to 500:1; the Chinese patent CN96109792.2 discloses a method for producing high quality Vaseline by serial hydrogenation process, wherein the volume ratio of hydrogen to oil is 300:1 to 1400:1; and the Chinese patent CN96120125.8 discloses a method for manufacturing white oil by means of direct hydrogenation from naphthenic base straight-run, wherein the volume ratio of hydrogen to oil is 500:1 to 1500:1.
These patents are characterized in a high volume ratio of hydrogen to oil, thus the hydrogen gas circulating step and the circulating hydrogen compressor are necessary.